The invention relates to a suction pipe for the intake air of internal combustion engines, which has a pressure-relieving site in the suction pipe for pressure waves coming from the cylinder.
Such pressure-relieving sites for suction pipes are generally known. The pressure-relieving sites are intended for the controlled dissipation of the energy contained in pressure waves due to so-called backfiring. Backfirings are flame reactions which, starting out from the cylinder-side inlets of the suction pipe, extend into the air-guiding ducts of the suction pipe. The danger that this phenomenon will occur exists particularly when the internal combustion engine is operating with a large timing overlap of the valves. The backfiring then consists of an already ignited portion of the fuel-air mixture in the cylinder. Backfiring can lead to destruction of the suction pipe structure, especially when plastic suction pipes are used. By these means, the functioning of the internal combustion engine is seriously endangered, since unfiltered supplementary air can be drawn in through the destroyed suction pipe. In this case, neither the supplying of the internal combustion engine with intake air of the required degree of purity nor the maintaining of limiting values with regard to the emission of pollutants by the internal combustion engine can be assured.
In order to eliminate this problem and thereby assure the emergency running properties of the internal combustion engine in the event of backfiring, a valve, for example, may be installed in the wall of the suction pipe. This comprises a rubber mushroom head, which assumes the function of a valve and is closed in the normal operating state of the internal combustion engine. In the event of backfiring, the pressure wave expands in the suction pipe and opens up the rubber mushroom head, which in this case functions as a pressure-relief valve. In this way, the overpressure in the surroundings of the internal combustion engine can be reduced, so that a component failure becomes less likely.
However, for reliable operation of the pressure-relieving site, an adequate cross section for the passage of the pressure wave is required. This cannot be provided by a rubber mushroom head valve, since the reliable operation of the valve cannot be assured when the dimensions of the component exceed a certain size. It would be possible to provide several valves. However, this represents an additional expense.
A further disadvantage of the pressure-relief valve has to do with flow properties. The geometry of the rubber mushroom head and of the associated outlet opening interferes with the internal contour of the suction pipe, as a result of which there is turbulence, which increases the flow losses of the intake air. Especially in the vicinity of the outlets of the suction pipe adjacent the cylinders, i.e. at the location where the backfiring originates, interference with the flow of the intake air is not acceptable, if a concerted production of turbulence in the combustion spaces of the cylinder is to be achieved. In other words, the rubber mushroom head must be mounted away from the location where the backfiring originates, as a result of which its effect is reduced.
A further disadvantage arises due to the use of a resilient or elastic material for producing a reasonably priced pressure-relief valve. The resilient or elastic properties are very dependent on the temperature, so that a constant functioning of the rubber mushroom head, independently of the operating temperature of the engine and of the external temperature, cannot be assured.